Light emitting diodes (“LEDs”) are p-n junction devices that have been found to be useful in various roles as the field of optoelectronics has grown and expanded over the years. Devices that emit in the visible portion of the electromagnetic spectrum have been used as simple status indicators, dynamic power level bar graphs, and alphanumeric displays in many applications, such as audio systems, automobiles, household electronics, and computer systems, among many others. Infrared devices have been used in conjunction with spectrally matched phototransistors in optoisolators, hand-held remote controllers, and interruptive, reflective, and fiber-optic sensing applications.
An LED operates based on the recombination of carriers (electrons and holes) in a semiconductor. When an electron in the conduction band combines with a hole in the valence band, it loses energy equal to the bandgap of the semiconductor in the form of an emitted photon; i.e., light. The number of recombination events under equilibrium conditions is insufficient for practical applications but can be enhanced by increasing the minority carrier density.
The minority carrier density is conventionally increased by forward biasing the diode. The injected minority carriers recombine with the majority carriers within a few diffusion lengths of the junction edge, generating photons at a wavelength corresponding to the bandgap energy of the semiconductor.
As with other electronic devices, there exists both the desire and need for more efficient LEDs, and in particular, LEDs that will operate at higher intensity while using less power. Higher intensity LEDs, for example, are particularly useful for displays or status indicators in various high ambient environments. High efficiency LEDs with lower power consumption, for example, are particularly useful in various portable electronic equipment applications. An example of an attempt to meet this need for higher intensity, lower power, and more efficient LEDs may be seen with the development of the AlGaAs LED technology for the red portions of the visible spectrum. A similar continual need has been felt for LEDs that will emit in the green, blue and ultraviolet regions of the visible spectrum which ranges from 400 nanometers (nm) (3.10 eV) to 770 nm (1.61 ev). Because red, green, and blue are primary colors, their presence is necessary to produce full color displays or pure white light.
As mentioned above, the wavelength (s) of photons that can be produced by a given semiconductor material is a function of the material's bandgap (Eg). This relationship can be expressed as s(nm)=1240/Eg(eV). Thus smaller bandgap materials produce lower energy, longer wavelength photons, while wider bandgap materials are required to produce higher energy, shorter wavelength photons. For example, one semiconductor commonly used for lasers is indium gallium aluminum phosphide (InGaAlP). This material's bandgap depends upon the mole or atomic fraction of each element present, and the light that InGaAlP can produce is limited to the yellow to red portion of the visible spectrum, i.e., about 560 to 700 nm.
In order to produce photons that have wavelengths in the green, blue or ultraviolet (LV) portions of the spectrum, semiconductor materials with relatively large bandgaps are required. Typical candidate materials include silicon carbide (6H—SiC with a bandgap of 2.5 eV) and alloys of indium nitride (InN with a bandgap of 1.9 eV), gallium nitride (GaN with a bandgap of 3.4 eV) and aluminum nitride (AlN with a bandgap of 6.2 eV). Since these nitrides can form solid solutions, the bandgap of these alloys (AlInGaN) can be tuned potentially from 1.9 eV to 6.2 eV with a corresponding wavelength varying from 653 nm to 200 nm at room temperature.